1. Field of the Invention
The present invention relates to a print head for an ink jet printer, wherein the print head comprises at least one ink supply channel and at least one nozzle with a nozzle channel and inflow opening, wherein ink is pressed out of the ink supply channel through the inflow opening into the nozzle channel and can be ejected from the latter, wherein the nozzle is arranged in a stationary manner on a side wall of the ink supply channel and to the at least one nozzle a ram is assigned with a ram end face in the ink supply channel which is spaced apart from the inflow opening, wherein the print head comprises first means for moving the ram end face in the ink supply channel between a reversal point that has a minimum distance from the inflow opening of the nozzle and a reversal point that has a maximum distance from the inflow opening of the nozzle.
Furthermore, the invention relates to a method for performing printing processes comprising the following steps:                providing a print head with ink supply channel, ram and nozzle with nozzle channel and inflow opening which forms the connection of the nozzle channel to the ink supply channel,        filling the ink supply channel with ink.        
2. Description of the Related Art
A print head for an ink jet printer, such as the subject matter of the invention, comprises an ink supply channel and at least one nozzle, wherein a movable ram is assigned to the nozzle for causing the ejection of ink from the ink supply channel.
First of all, some of the terms used in this document should be defined: the term “position of rest” is used in the present context to mean that a sealing body adopts a position in the ink supply channel of a print head that is dependent on the printing process, which means that no ink exits from the print head corresponding to printing a substrate.
The term “operating position” is defined in this context to mean that a sealing body adopts a position in the ink supply channel that is dependent on the printing process which enables the printing of a substrate with ink.
The term “means” is defined in this context to mean that according to the context either the singular or plural of the term may be intended.
The term “pigments” is defined in this context to mean particles in the ink with solid body properties that are not soluble.
The term “ink channel” is defined as a synonym for the term “ink supply channel” in this context.
The ink jet printing technique is a widely used printing technique for printing substrates. The print heads of ink jet printing devices generally comprise at least one ink supply channel and at least one nozzle for ejecting ink from the ink supply channel.
In the case of piezo-ink jet printers at least one piezo-element is deformed by applying an electric voltage, such that by means of the deformation a pressure wave is produced in the ink chamber or the ink channel, which causes the ejection of an ink droplet through the nozzle.
A print head of the said kind is known for example from WO 2008/044069 A. In the known device the nozzles, which are in the form of narrow strips with a passage opening for ink, are moved or set into oscillation in order to cause the ejection of a droplet from the nozzle.
In other known print heads, which enable the printing of high viscosity ink, for the ejection of droplets usually the ink is placed under excess pressure and a valve is opened briefly to let a droplet through. Such systems have the disadvantage however that when using inks with large sized pigments problems can occur with the tightness of the valves and it is very difficult to prevent sedimentation in the region of the valve seat.
Conventional print heads, which contain an ink supply channel and at least one nozzle, wherein a sealing body is assigned to each nozzle, comprise means which open a nozzle during the printing process. In the position of rest the nozzle is sealed tightly by the sealing body thereby preventing the ink pressurized with excess pressure from running out of the ink supply channel. In operating position the ram is lifted from the nozzle, so that ink can flow into the nozzle and can be ejected from the ink supply channel.
A print head of this kind is disclosed in document EP 0 445 137 B1. The document describes a print head for an ink jet printer with an ink chamber connected to an ink pressure source, in which a plurality of sealing bodies each closing a nozzle are arranged, which are each connected to a connecting rod and the sealing bodies are moved back and forth by a drive device in the ink chamber. In the position of rest the sealing body seals the ink jet nozzle completely. If the sealing body is moved from the position of rest into an operating position the latter is lifted from the nozzle or withdrawn. The ink is pressurized continuously with excess pressure in the ink chamber, so that only when removing the sealing body can ink be ejected from the ink supply channel through the nozzle. As soon as the sealing body has returned to the position of rest the ink jet nozzle is closed.
A further print head is disclosed in document EP 0 787 587 B1. The document describes a sealing body, which is formed to be T-shaped by a piston with an axially assigned sealing pin. The sealing body is located inside a cylindrical chamber, wherein the external diameter of the cylindrical piston corresponds approximately to the internal diameter of the chamber, so that the piston is moved in a sealing manner back and forth along the chamber wall. The piston separates the chamber into two areas, wherein one area comprises at the bottom a front plate which comprises a nozzle in the form of a borehole for ejecting ink droplets. Said area comprises the ink and forms the ink chamber, which is connected to an ink pressure source. In the other chamber is a spring, which pushes against the sealing body. According to the construction in the position of rest the sealing pin extends into the borehole of the nozzle and seals the latter, whereby an ink film is provided between the front plate and piston. If excess pressure builds up in the ink chamber, the pressure means that against the restoring force of a spring the piston and thus the sealing pin are removed from the borehole from the position of rest into an operating position, after which the ink pressurized by excess pressure flows into the borehole and can be ejected out of the nozzle. If the excess pressure drops the sealing pin is returned to the position of rest and moves back into the borehole, whereby the residual ink in the nozzle is displaced and the borehole is sealed.
Conventional print heads for performing printing processes with a sealing body are designed in their functional and characteristic structure for performing printing processes at relatively low printing frequencies which is reflected by relatively slow printing processes.
When performing a printing step the sealing body is moved from the position of rest into an operating position, wherein the ink pressurized with excess pressure exits from the ink channel through the nozzle. The ink flow is stopped in that the sealing body impinges in the nozzle, on the nozzle or on the inner wall of the ink chamber and tightly seals the inflow opening of the nozzle.
In order to separate an ink droplet cleanly from the residual ink, the sealing body has to close completely or seal the inflow opening of the nozzle, whereby a collision of the sealing body with the nozzle and/or the internal wall of the ink chamber is unavoidable. If the inflow opening of the nozzle in a position of rest is not completely sealed by the sealing body ink can escape continuously in the form of an ink jet out from the ink channel through the nozzle.
The more droplets required per unit of time for producing a desired pattern or structural print pattern, the higher the corresponding printing frequency of the printing steps to be performed.
Of course, the implementation of printing steps with a relatively high printing frequency is limited by periodic collisions of the sealing body with the nozzle and/or the internal wall of the ink chamber, as the collisions in the briefest period can cause the material failure of the sealing body and/or the nozzle which is associated with a short lifetime of the print head.
The stability of the printing process is reduced increasingly in conventional print heads the higher the printing frequency, as the probability of the material failure of the sealing body and/or nozzle increases accordingly.
In other conventional print heads, such as e.g. in document EP 0 787 587 B1, it is possible to avoid a hard collision of a T-shaped sealing body with the nozzle and/or internal wall of the ink chamber at least partly, as an ink film separates at least a section or part of the sealing body from the nozzle. If the sealing body is moved from an operating position into a position of rest, in the position of rest the sealing pin of the sealing body reaches into the channel of the nozzle and seals the latter, wherein however because of the construction and process the ink or ink film no longer displaceable in the ink chamber in a section opposite the end face of the sealing body piston cannot escape radially outwards over the sealing body piston external edge, as the sealing body piston is moved back and forth in a sealing manner on the chamber wall so that a collision of the sealing body with the ink film still occurs. A collision with the ink film is slightly gentler than a collision with a solid body, since as already known a fluid has greater compressibility than a solid body, so that the lifetime can be increased at least partly. However, material failure cannot be excluded but only delayed. In the known embodiment of the print head the following applies, i.e. the greater the printing frequency and thus the collision frequency, the greater the probability of material failure. If the ink contains pigments the failure of the material is even more likely.